Wafer sorters or wafer handling systems are commonly used in the manufacture and fabrication of various semiconductor devices. Such systems are used to automatically monitor wafer positions in cassettes and/or randomize wafers within the cassette between one process or testing operation station and another without human contact. Typically, a wafer handling system may employ a wafer handling robot or another such handling system that loads wafers into or out from a wafer carrier called a wafer cassette. A wafer cassette may hold an entire manufacturing lot of up to 25 wafers within evenly spaced slots of the cassette.
Once loaded, wafer cassettes may also be used as a carrier for the lot of wafers between the process or test operation stations to support the wafers without damage. Various wafer cassette designs are used, but typically the cassettes utilize wafers having a single wafer diameter. Some current wafer sorters pick wafers from or place wafers into the cassette by the use of a vacuum assisted end effector mounted on the wafer handling robot. The vacuum securely holds the bottom surface of the wafer down to the top of the end effector during the pick and place or sorting operations.
During process or testing operations, if a wafer is found to be sufficiently damaged, the wafer may be sorted out of the lot or otherwise identified by the process system for rejection. Various fixed sensors around a wafer cassette may sometimes be used to identify missing wafers or wafers protruding from the cassette but may not provide much, if any, information about the condition of the cassette.
Currently, in the fabrication of semiconductor devices, there is an ever increasing trend and need for greater process yield. Process yield has been found to be impacted by wafer scratches and other damage caused by the mishandling of the wafers by the wafer sorter or other processing equipment. In some cases, the root cause of such scratched or damaged wafers is that the wafer cassette has become warped, compressed, tilted or otherwise deformed. Wafer cassettes may become deformed due to thermal exposure, UV exposure, normal wear, crushing, prolonged or excessive stacking, dropping, or various other causes of damage or deformation. Such deformed wafer cassettes may result in wafer slots which are out of position relative to those which are expected (taught positions) by the sorter robot, for example, and may therefore cause a wafer to be dropped, scratched by the end effector, scratched by the wafer cassette, scratched by another wafer, or otherwise damaged or mishandled.
In addition, manufacturing variations between wafer cassettes, may contribute to the above mentioned deformation of wafer cassettes. Although one prior art cassette checking system has been attempted, this tester only verified the flatness of the lower surface of the cassette housing in a separate off-line test.
Accordingly, there is a need for a system and method for the measurement, evaluation, and disposition of wafer cassettes based upon evaluation of the measurement data obtained in-line during wafer sorting operations in the manufacture of semiconductor products.